1. Field of the Invention
This invention relates to an output sensor of a charge transfer device such as a CCD sensor.
2. Description of the Related Art
As a typical example of an output sensor of a charge transfer device such as a CCD sensor, an output sensor with a floating diffusion layer, named as a "floating diffusion amplifier", is known, as is disclosed in "High-Resolution 8 mm CCD Image Sensor with Correlated Clamp Sample and Hold Charge Detection Circuit", IE.sup.3 Transaction on Electron Device, Vol. ED-33, No. 6, June 1986. The floating diffusion layer is provided at the succeeding stage of the output gate of a charge transfer stage, and a wiring layer derived out from the floating diffusion layer is connected to a source follower circuit, for example. When a signal charge is transferred into the floating diffusion layer through the output gate, the potential thereof is changed, and the potential variation is converted into a signal voltage and supplied as an output signal voltage by means of the source follower circuit. After the signal charge of the floating diffusion layer is sensed, it is drained or discharged into the reset drain by turning ON the reset gate.
The sensitivity of the output sensor using the floating diffusion layer is determined by the parasitic capacitance associated with the floating diffusion layer. If the parasitic capacitance is large, the potential variation due to the signal charge becomes small so that the sensitivity will be set small the parasitic capacitance associated with the floating diffusion layer includes a junction capacitance inherent to the floating diffusion layer, a capacitance of the output wiring layer connected to the floating diffusion layer and an input gate capacitance of the source follower circuit connected to the output wiring layer. In general, the junction capacitance inherent to the floating diffusion layer is the largest one of these capacitances. Therefore, in order to attain a high sensitivity, it is desirable to suppress the area of the floating diffusion layer to a minimum. However, it is necessary to form a contact hole in the floating diffusion layer for connection between the output wiring layer and the floating diffusion layer and provide a sufficient space between the floating diffusion layer and each of the output gate and reset gate arranged on both sides thereof, the area of the floating diffusion layer cannot be freely reduced. For this reason, it is difficult to attain the high sensitivity in the signal charge sensor utilizing the floating diffusion layer.
An output sensing device which can be used instead of the output sensing device using the floating diffusion layer is proposed (IEEE Transactions on Electron Devices, vol. ED-27, No. 2, Feb. 1980). In the proposed output sensing device, an output sensor is formed of a MOS transistor having a surface channel region which has a conductivity type opposite to that of the buried channel region of a CCD, which is formed in the surface area of the buried channel region and which extends in a direction perpendicular to the charge transfer direction of the buried channel region. In a case where the output sensing device is used in an n-type buried channel CCD, a surface channel type p-channel MOS transistor is formed which has source and drain regions formed on both sides of the buried channel at the succeeding stage of the output gate of the buried channel. When a signal charge transferred via the buried channel is further transferred to under the surface channel region of the charge sensing MOS transistor via the output gate, the MOS transistor is substantially applied with a substrate bias voltage, modulating the conductivity of the surface channel region. In this way, the signal charge can be converted into current and externally supplied as a current output signal. In the output sensing device, the capacitance of the output section can be sufficiently reduced and the signal charge can be sensed with a high sensitivity in comparison with the output sensing device using the floating diffusion layer.
However, in this type of output sensing device, it is necessary to solve some problems in order to sense a minute signal charge with a high sensitivity. Since, in the signal charge sensing MOS transistor used in this output sensing device, the surface area of the buried channel region is used as a channel and the internal portion thereof into which the signal charge is injected is used as a gate electrode, the pn junction between the buried channel region and the substrate has a large input capacitance. Further, a large capacitance is also provided between the gate electrode and the buried channel region.
Thus, in the conventional output sensing device described above, it is still difficult to sense a minute signal charge with a high sensitivity.